The present invention relates generally to the removal of fine particles in biomass applications and, more specifically, to a selective catalytic reduction (SCR) reactor assembly for removing fine particles that interfere with SCR catalyst activity in biomass fuel applications.
The use of biomass fuel is considered an important option in mitigating the production of carbon dioxide (CO2) emissions from generating units designed to fire conventional fossil fuels. Although there is no generally accepted definition of biomass fuels, most observers consider organic waste streams and energy crops to comprise this category of fuels.
The key attraction of biomass fuels is that they are considered carbon neutral—the CO2 released by combustion was fixed or removed from the atmosphere, by photosynthesis, so its return does not provide a net carbon addition. The International Energy Association (IEA) has explored the co-firing of various types of biomass fuels, assessing potential benefits in numerous European countries. Specifically, the IEA and other research organizations in the European community have explored biomass co-firing in Central European countries such as Germany, the Netherlands, and Belgium where the energy infrastructure evolved based on broad availability of fossil fuels. In many cases co-firing biomass fuels also contributes to reducing combustion emissions of SO2, NOx, and particulate matter.
One potential downside to co-firing biomass with coal is the compromise to the performance of selective catalytic reduction (SCR) NOx control systems, specifically through accelerated deactivation of catalyst. Depending on the degree of catalyst deactivation, and the cost impact of accelerated catalyst replacement, a compromise to the effectiveness of SCR NOx control could limit biomass co-firing. Within the U.S., a total of approximately 112 GW of coal-fired generating capacity is presently equipped with SCR NOx control, and more than 120 GW is anticipated by 2012. Many of these SCR-equipped units are candidates for co-firing biomass fuels. The prospect of co-firing these units with biomass fuels and compromising NOx control, or significantly increasing in catalyst replacement, could limit the applicability of this CO2-mitigating technique to SCR-equipped units.
Table 1 compares the key features of several categories of solid biomass fuels to coals. The biomass fuels considered in this table are various wood products and wood residues, as well as grasses such as willows and straw. Agricultural byproducts such as olive residues are also shown.
Several features distinguish biomass fuels from coals. The relatively high moisture content is perhaps the most notable feature, and comprises a disadvantage as it lowers heating value of the fuel. The ash content of biofuels varies over a very wide range, from as high as 20% to less than 1%. Biomass fuels contain low concentrations of sulfur and nitrogen compared to coal, but can have higher chlorine. Notably, the content of trace elements such as strong alkali (e.g., potassium) can be several orders of magnitude higher compared to most coals.
Also, as shown, the potassium content of most biomass fuels is orders of magnitude higher than in coal. Perhaps more important than the quantity of potassium is the physical and chemical form. In contrast to coal where potassium exists mostly as a metal oxide, biomass fuels contain potassium as an organically bound salt, usually potassium chloride (KCl) or sulphate (K2SO4). This form of potassium enables volatilization easier than if the potassium exists as a metal oxide. Further, this form of potassium within biomass fuels allows fine particles to be generated, which are the major poisons for SCR catalyst.
TABLE 1ForestReedResiduesCanaryWood(coniferousGrasswithouttree with(SpringOlivePropertyCoalPeatBarkBarkneedles)WillowStrawharvested)ResiduesAsh Content, % (d) 8.5-10.94-70.4-0.52-31-31.1-4.056.2-7.52-7Moisture 6-1040-55 5-6045-6550-6050-6017-2515-2060-70Content, %Net Calorific11,178-12,1678,985-9,1577,954-8,5987,954-9,8887,954-8,5987,954-8,2557,4817,352-7,5247,524-8,168Value, Btu/lbCarbon, % (d)76-8752-5648-5248-5248-5247-5145-4745.5-46.148-50Hydrogen, % (d)3.5-5    5-6.56.2-6.45.7-6.8  6-6.25.8-6.75.8-6.05.7-5.85.5-6.5Nitrogen, % (d)0.8-1.51-30.1-0.50.3-0.80.3-0.50.2-0.80.4-0.60.65-1.040.5-1.5Oxygen, % (d) 2.8-11.330-4038-4224.3-40.240-4440-4640-464434 Sulfur, % (d)0.5-3.1<0.05-0.3 <0.05<0.05<0.050.02-0.100.05-0.2 0.08-0.130.07-0.17Lead, % (d)<0.10.02-0.060.01-0.030.01-0.030.01-0.040.01-0.050.14-0.970.09  0.1*Potassium, % (d)0.0030.8-5.80.02-0.5 0.1-0.40.1-0.40.2-0.50.69-1.3 0.3-0.530*Calcium, % (d) 4-120.05-0.1 0.1-1.50.02-0.080.2-0.90.2-0.70.1-0.69
Early work in the role of alkali compounds poisoning SCR active sites demonstrated that stronger alkali such as sodium and potassium were key. In particular, it was found that potassium asserted a significant impact on catalyst activity. Further research conducted to diagnose biomass-derived deactivation, has focused on not only the composition of alkali materials, but the physical form, specifically, submicron particles. As examples, work conducted by Kling, Larsson, and Zheng have addressed not just the composition, but particle size of materials that form aerosols, and are believed to be key in prompting catalyst deactivation.
The effect of biomass on SCR catalyst activity entails the following three steps: (a) evaporation and volatilization of organically bound minerals and salts within the furnace or combustion system, (b) condensation of these vaporized species forming small aerosol particles, (c) deposition of aerosols on the catalyst surface, and (d) diffusion and reaction of aerosols within the catalyst, specifically the pores. The first two mechanisms are not relevant to the present invention and will not be further discussed. The latter two mechanisms are discussed as further background material for the present invention.
The deposition of predominantly potassium-containing aerosol particles on the surface of SCR catalyst has been observed in bench-scale tests. More specifically, fouling by K2SO4 and KCL submicron particles has been observed in laboratory bench-scale reactors. The results suggest that such aerosol particle fouling is more severe than in coal-fired applications due to the small size of the particles and their susceptibility to Brownian and turbulent diffusion. The absence of large particles in the flue gas is also a factor. As KCl and K2SO4 have lower melting points than typical coal ash, the greater stickiness may also contribute to the rapid accumulation. These measurements strongly implicate potassium-based compounds—specifically KCl and K2SO4—as key contributors to SCR catalyst deactivation.
The consequence of this observation, in terms of mitigating countermeasures, is that submicron aerosols are highly mobile in defusing to the catalyst surface. The presence of large particles that obstruct this diffusion or of laminar-line conditions that minimizes mass transfer coefficients are favored.
Mechanistically, Zheng showed by SEM-EDX measurements that potassium compounds penetrate into the catalyst wall, and further that Bronsted acid sites critical for catalytic activity reacted with potassium and were rendered inactive.
The penetration of KCl to the active site is more prone to poisoning than K2SO4. Zheng suggests that submicron particles are required for potassium penetration. These general observations—that alkali submicron particles generally less than 100 nm penetrate into catalyst pores and are key behind deactivation—are supported by observations by Kling.
Of note in these observations is the susceptibility of the catalyst to submicron particles, the latter driven by Brownian motion. In coals, the potassium and other inorganic minerals are associated with minerals, whereas in biomass, alkali metals are associated with salts or part of the organic matrix. The latter are more easily released to the fuel at combustion temperatures.
At present, there are no countermeasures to the deactivation of SCR catalyst by biomass fuels, as imparted by the submicron particles. Some attempts have been offered to change the form of the offending potassium submicron particles to a state that is less soluble, but these to date have not been successful.
Accordingly, a simple approach to remove fine particles and prevent their penetration into the catalyst surface is needed.